In 3rd Generation Partnership Project (3GPP) which is a mobile communication system standardization project, a technique of efficiently utilizing a specific base station (for example, a small cell base station) having a narrower coverage than a general base station (for example, a macro cell base station) is under review.
In 3GPP, a dual connectivity scheme (dual connectivity) is scheduled to be introduced in Release 12 or later (see Non Patent Literature 1). In the dual connectivity scheme, a user terminal simultaneously establishes a connection with a plurality of base stations (a general base station and a specific base station). Since radio resources are allocated from each of the base stations to the user terminal, the throughput is expected to be improved. The dual connectivity scheme is also referred to as inter-base station carrier aggregation (inter-eNB CA).
In the dual connectivity scheme, only one base station (hereinafter referred to as a “master base station”) among a plurality of base stations establishing a connection with the user terminal establishes an RRC connection with the user terminal. On the other hand, other base stations (hereinafter referred to as “secondary base stations”) among a plurality of base stations provide additional radio resources to the user terminal without establishing the RRC connection with the user terminal.